String-corrected black holes

We investigate the geometry of four dimensional black hole solutions in thepresence of stringy higher curvature corrections to the low energy effectiveaction. For certain supersymmetric two charge black holes these correctionsdrastically alter the causal structure of the solution, converting seeminglypathological null singularities into timelike singularities hidden behind afinite area horizon. We establish, analytically and numerically, that thestring-corrected two-charge black hole metric has the same Penrose diagram asthe extremal four-charge black hole. The higher derivative terms lead toanother dramatic effect -- the gravitational force exerted by a black hole onan inertial observer is no longer purely attractive! The magnitude of thiseffect is related to the size of the compactification manifold.Comment: 22 pages, 6 figures, harvmac, eps